The neural retina is the key organ involved in transducing light into electrical signals that deliver information to the brain, and degeneration of this tissue is a leading cause of irreversible blindness. Common retinopathies include degeneration of retinal ganglion cells which comprise the optic nerve (i.e., glaucoma), light-sensing photoreceptor cells (i.e., macular degeneration or retinitis pigmentosa), or retinal pigment epithelia (RPE) which maintain photoreceptor health and function and whose degeneration results in a variety of retinopathies. We will highlight here recent advances in the development of retinal cell-replacement therapies, focusing on the various cell types which have been used for photoreceptor cell replacement (Fig. 1).
Currently, retinal degenerative diseases are incurable and retinal degeneration is irreversible. Therapies to treat retinal dystrophies are limited to treatments which strive to delay the onset or progression of degeneration, but no therapies are available to replace lost photoreceptor cells and restore vision. Stem cells in various adult tissues continuously generate terminally differentiated functional cells to replenish worn out or damaged cells. Major efforts are ongoing toward identifying an endogenous retinal stem cell population in mammalian eyes with the hope that they can be either activated in vivo to regenerate injured or degenerated retinal cells or cultured in vitro for generation of differentiated retinal cells for transplantation (Fig. 1).
A population of retinal stem cells in the ciliary body of mouse and human eyes were initially identified and claimed to have the ability to be expanded in vitro and produce all of the retinal cell types.1–3 However, later studies questioned the findings by showing that these cultured retinal stem cells from the ciliary body are retinal pigmented epithelial cells and also lack the ability to generate the differentiated retinal cell types, particularly photoreceptor cells.4,5 Retinal stem cells can be isolated from porcine and murine retina and can restore visual function after allogeneic retinal transplantation.6,7 In fish and chicken eyes, Müller cells can proliferate and generate differentiated retinal cell types in response to injury.8 Müller cells may also function as retinal stem cells in mammalian eyes. In mouse and rat, neurotoxic injury can activate Müller cells in vivo and induce expression of progenitor markers9 and repopulate various types of retinal cells, such as amacrine cells,10 bipolar cells, and rod photoreceptors.11 Müller cells isolated from mouse, rat, and human can be expanded in vitro and differentiated along various retinal lineages, the process of which is regulated by Wnt and Notch signaling12,13 and can be used for retinal transplantation.14,15 Future studies will be needed to determine whether endogenous Müller cell-mediated repair mechanisms can be stimulated for therapeutic use, but until that time patients suffering from various retinal degenerative diseases and dystrophies will benefit most from continued development of cell-replacement therapies.
Progenitor populations isolated from the eye or the brain may be a potential source of material for retinal transplantation (Fig. 1). Early studies demonstrated successful integration of adult rat hippocampal-derived stem cells into the neonatal16 and mature rat retina.17–19 Progenitor cells isolated from embryonic murine6,20 and porcine7 retina have also been successfully transplanted into the subretinal space of recipient mice. Other studies demonstrate that transplanted retinal progenitor cells isolated from human adult eye can successfully integrate into recipient mouse retinal tissue.2 All of these studies have relied on the in vivo ocular environment to carry out terminal differentiation of the donor progenitor cells into mature photoreceptor cells. However, successful transplantation and integration of freshly-isolated, post-mitotic but not terminally-differentiated neonatal murine rod photoreceptors into the subretinal space of mice21 demonstrated that progenitor cells that are committed to photoreceptor fate but immature are capable of increased integration into recipient retina tissue when compared to mitotic progenitors. The cells integrated into the outer retina, formed mature photoreceptors based on morphology and expression of photoreceptor markers, and restored visual function. These studies are important for establishing the principle of cell-replacement therapy; the recipient retina is indeed receptive to cells from an external source, and the retinal environment can support and maintain foreign cells. However, freshly-isolated, post-mitotic but immature retinal cells are difficult to obtain and be expanded and thus may not be an ideal source of donor material. In order to translate this technology into a clinical application, much focus has been applied to the derivation, rather than isolation, of donor material in vitro.
In the absence of suitable strategies for isolation and expansion of specific cell types for human retinal transplantation, recent focus has been applied toward the in vitro production of retinal cells (Fig. 1). Human embryonic stem (hES) cells exhibit unlimited self-renewal and pluripotency and thus represent a potential expandable source of photoreceptor cells, if directed down the appropriate differentiation pathway. Indeed, hES can be differentiated in vitro toward a retinal cell fate22–24 and transplanted into the subretinal space of adult mice,25 where expression of key photoreceptor markers including Rhodopsin and Recoverin was observed and visual function was restored to vision-impaired Crx−/− mice.
Though many ES cell lines have been established, the use of embryonic material is controversial and heavily regulated, and it may not be an ideal source of donor material. In order to circumvent ethical and regulatory limitations of the use of human embryonic material, recent focus has been placed on the genetic modification of adult somatic cells to generate retinal cells. This area of research includes direct reprogramming of adult human fibroblasts to retinal progenitor/mature retinal cells, reprogramming of adult fibroblasts to an ES cell-like state termed inducible pluripotent stem (iPS) cells,26,27 and their subsequent differentiation to the retinal lineage. iPS cells possess the ability to differentiate into retinal cells in vitro and can successfully integrate and express photoreceptor markers upon transplantation into the mouse retina.28–31 Reprogramming of adult fibroblasts to a neural progenitor state has also been achieved32 and could potentially be used for transplantation into the neural retina. At this time, reprogramming of adult fibroblasts to retinal progenitors or mature retinal cells remains to be demonstrated. In addition to enabling improvements in the ease of generation and abundance of transplantable material, direct reprogramming and reprogramming via iPS cell intermediates of fibroblasts to retinal cell types would allow the derivation of patient-specific donor material. Allogeneic transplantation would avoid potential complications due to immune rejection.
Significant progress has been made in the past decade toward the development of safe and effective cell replacement-based therapies for the treatment of retinal degenerative diseases. This was exemplified in 2011 when Advanced Cell Technology Inc. was approved to conduct clinical trials using hESC-derived RPE cells to treat age-related macular degeneration and Stargardt macular dystrophy. This project signifies the inaugural clinical trial utilizing human ES cell-derived material for treatment of disease and provides hope that cell-based therapies may safely and effectively be used for treatment of retinal dystrophies in the future.
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